1. Field of the Invention
The present invention relates to focus detection operations of a focus detection device. More particularly, the invention relates to a focus adjustment device that eliminates unnecessary driving of a shooting lens after the shooting lens has already been driven near to an in-focus position.
2. Description of Related Art
One of the known focus detection methods of an autofocus detection device in a camera is the phase difference detection method. In this method, two images of an object with parallax are guided to a pair of photo-electric conversion element arrays. An in-focus condition is determined by computing the relative shift amount of the two images based on an image output from each photo-electric conversion element array.
Hereafter, this phase difference detection method will be explained with reference to FIG. 4. Light rays passing through region 21 of a shooting lens 1 focus on a film equivalent plane 6. The light rays then pass through, in order, a band pass filter 7, a vision mask 2, a field lens 3, a stop aperture unit 41, and a re-imaging lens 51 of a focus detection optical system 8 and form a secondary image on a sensor array 9A of an image sensor 9. Similarly, light rays passing through region 31 of the shooting lens 1, after focusing on the film equivalent plane 6, pass, in order, the band pass filter 7, the vision mask 2, the field lens 3, a stop aperture unit 42, and a re-imaging lens 52 of the focus detection optical system 8 and form a secondary image on a sensor array 9B of the image sensor 9.
A focus detection region is a section of the assembly near a predicted focus plane where the sensor arrays 9A and 9B of the image sensor 9 overlap each other, with the image being invertedly projected by the focus detection optical system 8.
The size of region 21 of the shooting lens 1 is the same as an inverted projected image formed by the field lens 3 of the stop aperture unit 41. Likewise, the size of region 31 of the shooting lens 1 is the same as an inverted projected image formed by the field lens 3 of the stop aperture unit 42. The secondary images of the pair of object images formed by the focus detection optical system 8 on sensor arrays 9A and 9B move away from each other in a so-called front focus condition when the shooting lens 1 forms a clear image of the object in front of a predicted focus plane. Conversely, the secondary images move towards each other in a so-called rear focus condition when the shooting lens 1 forms a clear image of the object to the rear of the predicted focus plane. The secondary images on the sensor arrays 9A and 9B relatively coincide with each other in a so-called in-focus condition when the shooting lens 1 forms a clear image of the object on the predicted focus plane.
Therefore, the focus detection condition of the shooting lens 1, that is, the amount and the direction of deviation from the in-focus condition, hereafter referred to as defocus amount, is determined by converting the pair of secondary images of the object formed by the focus detection optical system 8 into electric output signals through photo-electric conversion by the sensor arrays 9A and 9B of the image sensor 9 and by obtaining relative positions of the pair of secondary images of the object through computation of the pair of object image signals.
A method is also known wherein the object image signals obtained by photo-electric conversion of the image sensor 9 are filter processed to extract high frequency and low frequency components of the object image signals. The defocus amount is computed by further processing the filter processed signals.
Next, the computational processing method to obtain the defocus amount will be explained. Each of the sensor arrays 9A and 9B of the image sensor 9 is structured in such a manner that a plurality of photo-electric conversion elements line-up in one direction. Each of the sensor arrays 9A and 9B output a plurality of photo-electric conversion outputs a.sub.1 . . . a.sub.n and b.sub.1 . . . b.sub.n corresponding to the number of photo-electric conversion elements in each array (See FIGS. 5a and 5b). Correlation is performed while shifting these data strings relatively by a predetermined amount of data. In particular, the following formula is used to compute the correlation amount C(L). ##EQU1##
Here, L is an integer representing the amount of shift of the data string, as described above, and the first term k and the last term r can be changed depending on the shift amount L. The defocus amount is computed by multiplying the shift amount yielding a relative minimum among the resulting correlation amounts C(L) and a constant determined by the pitch width of the photo-electric conversion element of the image sensor and the optical system described in FIG. 4.
However, the correlation amounts C(L) assume discrete values, as described in FIG. 5(c), and the minimum unit of detectable defocus amount is restricted by the pitch width of the photo-electric conversion elements of the sensor arrays 9A and 9B of the image sensor 9. Hence, a method is proposed (see Japanese Patent Publication No. Sho 60-37513) in which more precise focus detection is executed by computing a new relative minimum for the discrete correlation amounts C(L) using an interpolation algorithm. In this method, the defocus amount is determined based on the relative minimum of the correlation amount C(0) and the correlation amounts C(1) and C(-1) computed from the shift amounts at both sides. A shift amount F.sub.m and a defocus amount DF yielding the relative minimum correlation amount C.sub.ex are determined by the following formulae. EQU DF=K.sub.f .times.F.sub.m EQU F.sub.m =L+DL/E (2) EQU DL=[C(-1)-C(1)]/2 EQU C.sub.ex =C(0)-.vertline.DL.vertline. EQU E=MAX[C(1)-C(0),C(1)-C(0) . . . ]
Here, MAX (C.sub.a, C.sub.b) denotes the larger of C.sub.a and C.sub.b, while K.sub.f is a constant determined by the pitch width of the photo-electric conversion elements of the optical system and the image sensor. FIG. 6 shows a graphical representation of some of these variables. The defocus amount DF thus obtained needs to be checked to determine whether it represents a true defocus amount or if it is the result of fluctuations of the correlation amount due to noise and the like. A defocus amount satisfying the following criteria is accepted as having a sufficient confidence level. EQU E&gt;E.sub.1 ( 3) EQU C.sub.ex /E&lt;G1
where E.sub.1 and G.sub.1 represent certain predetermined values respectively. E is a value that depends on the contrast of the object. As the value of E becomes larger, the contrast becomes larger and the confidence level becomes higher. C.sub.ex /E depends on the coincidence level of the images, and the closer it is to 0, the higher the confidence level becomes. When a defocus amount is accepted as having a sufficient confidence level, the shooting lens 1 is driven based on the determined defocus amount DF. This explains the phase difference detection method.
The defocus amount of the shooting lens is detected repeatedly by executing the focus detection operation repeatedly using the above-stated focus detection method. As described above, once the defocus amount detected is accepted as having a sufficient confidence level, the shooting lens is driven based on the detected defocus amount until a defocus amount is obtained indicating that the focus condition of the shooting lens is in-focus or in a range considered to be in-focus.
Moreover, by repeating the focus detection operation, even after the defocus amount is determined to be in an in-focus condition or in a range approximately at the in-focus condition, driving of the shooting lens is resumed until a defocus amount is reached indicating that the shooting lens is determined to have exceeded the defocus amount indicating the in-focus or approximately in-focus condition. By continuing the focus detection operation in this manner, the shooting lens is always maintained in the in-focus condition.
Now, in the focus detection operation described above, the following problems occur even though the shooting lens is quick to focus on the object. For example, if the object to be focused temporarily moves out of the focus detection region due to shaking of the camera and the like, the shooting lens is driven to focus on another object (for example, the background) in the focus detection region, and then the shooting lens is re-started when the original object enters the focus detection region again. Shaking occurs easily, especially when using telephoto lenses. The fact that the shooting lens focusing is started immediately when the desired object leaves the focus detection region is not desirable since it causes a loss of picture taking opportunities. Moreover, if another object crosses between the desired object and the camera, the lens is driven to try to focus on the other object at the moment it enters the focus detection field. Then the shooting lens is re-started with an attempt to focus on the original object at the moment the other object leaves the focus detection region, causing a similar problem as above.